ECE 3574: Applied Software Design Thread Safe Queue Today we are - PowerPoint PPT Presentation

ECE 3574: Applied Software Design Thread Safe Queue

Today we are going to look in detail at how to make a data structure thread-safe. ◮ Review of std::queue ◮ push ◮ empty ◮ try_pop ◮ wait_and_pop ◮ Message Queues ◮ Exercise

Review std::queue A first-in-first-out queue with (basic) methods ◮ push ◮ pop ◮ empty

Like all standard containers, std::queue is not thread-safe ◮ Q: How can we adapt the queue to protect access? ◮ A: mutexes and condition variables We protect each method with a mutex.

The interface template < typename T> class ThreadSafeQueue { public : void push(const T & value); bool empty() const; bool try_pop(T& popped_value); void wait_and_pop(T& popped_value); private : std::queue<T> the_queue; mutable std::mutex the_mutex; std::condition_variable the_condition_variable; };

Simplest case: empty member function template < typename T> bool ThreadSafeQueue<T>::empty() const { std::lock_guard<std::mutex> lock(the_mutex); return the_queue.empty(); }

push member function template < typename T> void ThreadSafeQueue<T>::push(const T& value) { std::unique_lock<std::mutex> lock(the_mutex); the_queue.push(value); lock.unlock(); the_condition_variable.notify_one(); }

try_pop member function No waiting, returns true on success, popped value as an output argument. template < typename T> bool ThreadSafeQueue<T>::try_pop(T &popped_value) { std::lock_guard<std::mutex> lock(the_mutex); if (the_queue.empty()) { return false ; } popped_value = the_queue.front(); the_queue.pop(); return true ; }

wait_and_pop member function Wait for available, returns popped value as an output argument. template < typename T> void ThreadSafeQueue<T>::wait_and_pop(T &popped_value) { std::unique_lock<std::mutex> lock(the_mutex); while (the_queue.empty()) { the_condition_variable.wait(lock); } popped_value = the_queue.front(); the_queue.pop(); }

Thread-safe queues are a good way to implement message passing between threads, where they are called Message Queues . ◮ Each thread has a pointer or reference to a shared input ThreadSafeQueue holding units of work ◮ Each thread has a pointer or reference to a shared output ThreadSafeQueue holding results of work ◮ Each thread calls wait_and_pop on input queue, does the work, then calls push on the output queue Often a single thread, the Producer , pushes into the input queue and pops from the output queue. The other threads act as Workers or Consumers .

Exercise See the website.

Next Actions and Reminders ◮ Read about Producer/Consumer Pattern